1. Field of the Invention
The invention relates generally to methods and systems for acquiring acceleration and/or velocity data using fiber optic sensors. Specifically, the invention relates to grating sensors with ultra narrow band gratings, combined with interferometric wavelength-to-phase conversion and low noise interferometric interrogation.
2. Background of the Invention
There are many applications that require a device to measure the dynamic acceleration or acoustic velocity signal at a given location. Examples include: the seismic exploration/monitoring of oilfields, seismic monitoring for earthquakes, structural integrity monitoring, and health monitoring of vibrating equipment/machinery acoustic monitoring in marine environments (e.g., SONAR). For decades, such monitoring has been almost exclusively performed using electronic-based sensors such as piezoelectric sensors and magnet/coil sensors. These sensors typically generate a voltage output that is proportional to the intensity of the applied vibratory motion (displacement, velocity or acceleration). Because the generated voltage levels are relatively weak (i.e., low level), electronics are required for amplification, signal conditioning, filtering, and in most cases digitization/multiplexing. These electronics must be located very close to the sensor to limit the introduction of noise into the system. Thus, the electronics must be designed to operate in the local environment (temperature/vibration/humidity/shock) where the sensor is placed.
Recently, the use of fiber optic sensors has become more prevalent for sensing applications, particularly in those applications where the sensors must be placed in harsh environments, which seriously affects the performance/reliability of the associated electronics. Fiber optic sensors have an advantage in that they require no electronics at or near the sensor. In fiber optic sensors, light is sent through the optical fiber from a remote location (in a benign environment). The measurand causes a change in the optical transmissive property of the fiber which is then detected as a change in the received light signal at the remote electronics.
Fiber optic sensors generally fall into two categories, those designed for making high speed dynamic measurements, and those designed for low speed, relatively static measurements. Examples of dynamic sensors include hydrophones, geophones, and acoustic velocity sensors, where the signal varies at a rate of 1 Hz and above. Examples of low speed (static) sensors include temperature, hydrostatic pressure, and structural strain, where the rate of signal change may be on the order of minutes or hours. This invention relates primarily to dynamic measurements of acceleration, acoustic velocity, and vibration using fiber optic sensors. Historically such sensors have been more costly than the legacy electronic versions because they are difficult to manufacture, require complicated and expensive equipment for even limited automated assembly, and involve significant amounts of skilled touch labor to produce. Although fiber Bragg grating (FBG) accelerometers are currently available, they incorporate spectroscopic interrogation, which limits the sensitivity to about 1 mg. However, many applications require sensitivities on the order of 30-50 ng. Fiber laser devices have also been used for sensing. However, they are expensive and tend to be unstable. The invention endeavors to solve these problems and more to provide extremely high sensitivity acceleration measurements suitable for a wide range of applications requiring sensors in environments in which electronics often cannot survive.